Numerous aircraft crashes have been attributed to false readings of aircraft flight velocity by pitot-static tube measuring devices mounted on the aircraft. A pitot-static tube senses aircraft velocity by measuring the difference between free stream total pressure and local static pressure; the difference between these two pressures being the dynamic pressure as known from Bernoulli's equation. Flight velocity is then computed from dynamic pressure using the air density calculated from knowledge of the atmospheric temperature and the static pressure; the temperature being measured independently.
An ordinary pitot-static tube sensor, as described in “Pitot-Static Tube-Prandtl Tube”, http://www.grc.nasa.gov/WWW/K-12/airplane/pilot.html, consists of a hollow tube, enclosed in an aerodynamic fairing, with the hollow inner tube leading to an inner chamber closed on its downstream end. The pitot-static tube sensor faces into the oncoming free stream flow with the air which enters the tube producing the free stream total pressure level after the air is decelerated to zero velocity in the inner tube and inner chamber of the pitot-static tube. The static pressure signal is obtained from a flush mounted static pressure port usually facing outward thru the side of the pitot-static tube. The dynamic pressure is measured in one embodiment of the ordinary pitot-static tube sensor by placing a pressure measuring transducer in the inner chamber with the free stream total pressure facing against one side of the pressure measuring diaphragm and with the static pressure on the other: the pressure difference across the diaphragm being the dynamic pressure which is equal to one half the atmospheric density times the free stream velocity (V) squared. The pitot-static tube is normally constructed from metal.
The velocity measuring pitot-static tube operates on the basis that it is operating properly if there is no obstruction of the free stream flow as it enters into or slows to zero velocity inside the hollow inner tube and inner chamber and no interference or blockage of the static pressure port providing the static pressure signal. In some pitot-static tube installations, the pitot-static tube is mounted on a vane which is free to rotate in pitch angle so that the pitot-static tube always faces directly into the oncoming flow. Early examples of patents granted on pitot static tubes (without the ice, sleet, frost, deicing fluid, rain water and snow detection capabilities of the present invention) in the period from 1927 thru 1940 include patents, for example, by Colvin in U.S. Pat. No. 1,620,633, by Peace, Jr. in U.S. Pat. No. 1,971,534, by Colvin in U.S. Pat. No. 2,042,700, by Diehl in U.S. Pat. No. 2,179,500, by Kollsman in U.S. Pat. No. 2,204,367.
U.S. Pat. No. 1,620,633 offers a pitot-static tube sensor constructed using two separate tubes strapped together for support: one that measures the free stream total pressure and the other measuring the free stream static pressure: with the two tubes supplying the pressures so measured to an airspeed transducer or airspeed indicator. In contrast, the pitot-static tube of U.S. Pat. No. 1,971,534 combines the measurement of free stream total pressure and free stream static pressure in a single tubular structure with two chambers, one chamber for each of the two pressures, and also equips the pitot-static tube with an electric heater to melt ice, frost or packed snow forming on the upstream open end of the pitot-static tube.
In U.S. Pat. No. 2,042,700, a single tubular structure with two chambers is also used with a baffle in its upstream end that prevents ice, frost or snow from entering the pitot-static tube and allows any melted snow or ice melted by an electric heater to run out the upstream end of the tube. An electric heater is provided in this patent so that the pitot-static tube may function properly when flying thru cold or inclement weather. In U.S. Pat. No. 2,179,500, the pitot-static tube incorporates heating of the pitot-static tube using electrical energy or heating thru the use of the aircraft's engine exhaust. In U.S. Pat. No. 2,204,367, drains are added to the pitot-static tube to allow water, which forms from ice, frost or snow melted by an electric heater or from incident rain, to drain overboard near the upstream end of the pitot-static tube. The pitot-static-tube is also equipped with a downstream trapping chamber with drain that traps and then drains any water that may make it that far downstream in the pitot-static tube.
Heating of the pitot-static tube to allow operation in icing, frost and snow conditions and the use of baffles and drains holes, as done in these early pitot-static tube patents, has continued to be features included in all pitot-static tubes used on present day commercial aircraft as well as included in the present invention.
More recently, pitot-static tubes have been mounted on vanes that rotate in response to aircraft pitching motions to keep the pitot-static tube facing at all times directly into the oncoming free stream flow. Examples of vane mounted pitot-static include patents by Collot et al in U.S. Pat. No. 6,679,112, by Collot et al in U.S. Pat. No. 6,817,240, by Hanson et al in U.S. Pat. No. 7,124,630 and by Collot et al in U.S. Pat. No. 7,155,968. No prior art has been found that equips a pitot-static tube with the ability to sense when ice, sleet, frost, deicer fluid, rain water or snow has attached itself to the exterior of the pitot-static tube or has partially or fully filled the inner entrance tube or has partially or fully filled the inner chamber or has partially or fully covered the static pressure measuring taps on the side of the pitot-static tube: information that the present invention provides when the Ice Detection pitot-static tube is used in fixed positions or mounted on rotatable vanes.
In a number of prior aircraft crashes, pitot-static tube velocity readings are suspected as having been false due to icing and are believed to have lead to the crash. One example of a crash, as described in “Air France Flight 447”, http://en.wikipedia.org/wiki/Air_France_Flight—447, believed to be due to ice collecting on or in one or more of the aircraft's pitot-static tubes during flight is that of Air France Flight 447 which on 1 Jun. 2009 crashed into the Atlantic Ocean on a flight from Rio de Janeiro, Brazil to Paris, France with the loss of 216 passengers and 12 crew members. In this accident, Air France Flight 447 was flying at a flight altitude of 35,000 ft (11000 m) and at a speed of 467 knots (865 Km/h/537 mph) just prior to the crash.
To date, claims of false pitot-static tube velocity readings leading to aircraft crashes are conjecture for there is no existing evidence that icing was indeed the cause. The absence of evidence is due to the fact that existing pitot-static tubes used on commercial passenger aircraft, as well as on all aircraft employing pitot-static tubes, are not instrumented for detecting the presence of ice, sleet, frost, deicer fluid, rain water or snow. Also, no prior experimental measurements are available which might provide an understanding of how ice buildups in flight on and in a pitot-static tube and how icing might effect, to some degree, the operation of a pitot-static tube velocity measuring device.
Reviews of prior commercial jet crashes from high altitudes have led to the tentative conclusion by the aviation community that pitot-static tube icing may be a common factor in these crashes. It is also noted in a recent review that pitot-static tube icing might be a factor in the crash of Air France Flight 447 for it was operating in a cold environment at 35,000 feet altitude which is a common factor with some of the previous crashes. However, no prior art was found to exist for a pitot-static tube with the ice, sleet, frost, deicer fluid, rain water or snow detection features of the present invention.